This is a passive 2 component filter. One inductor in series with a 500Ω resistor. The total is connected in parallel with the ribbon mic output transformer. It should be a low inductance coil for this application. So at most frequencies the coil is almost short circuit. So one is left with the resistor of 500Ω. If the output impedance at the transformer is 500Ω we have half voltage drop ie a drop of 6dB at all those frequencies. But above a turnover frequency the reactance of the inductor increases so the output of the mic increases.
By playing with values of L and R we can do anything we want.
On the RCA ribbon mic described by Dr. Olson, top executive of the Acoustical and Electromechanical Research Laboratories of RCA, there is also a switchable inductor across either the full secondary or half of it. This is a Pleiades filter for the switchable settings V1, voice 1 or V2, voice 2 for compensating for the proximity effect.
Similar L or RL, or RLC circuits can be seen inside other legendary mics such as Electro-Voice, Shure etc. An excellent source of info and schematics for each mic is www.coutant.org. Our grandfathers new that simplicity in the signal path leads to excellent sound quality and we can hear this quality on the legendary recordings that they have left us.
Does the MD441 use similar circuits across the voice coil?
Inductors and transformers sound excellent and natural. Usually inductor cores are made by winding ring or toroidal magnetic (tape wound) cores. Materials that have been used are MolyPermalloy (molybdenum permalloy), mu metal, Nanocrystalinne Nanoperm (www.magnetec.de) etc.
The above mentioned very high frequency boost circuit should normally be not needed due to the Fletcher-Munson and voice effort curves.
But should you need to reproduce a glockenspiel or an orchestra for example at much lower acoustical intensity than when tracking them it is may be useful. See also a nearby euroelectron post that references the same paper by Dr. Olson. on the paper by Dr. Harry Olson.
In other cases the mic may sound worse as now the frequency responce to listener's brain may not be flat in many situations.
References:
Ribbon Velocity Microohones - Harry F. Olson - JAES - June 1970 - volume 18 - number 3
(Flat frequency response from the vocal chords of the singer-actor to the brain of the listener) - Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Society of Motion Picture Enginners
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